The device is repeatedly fired every 4 seconds over the period when the storm is approaching & until it has passed through the area. What would otherwise have fallen as hail stones then falls as slush or rain. It is critical that the machine is running during the approach of the storm in order to affect the developing hail stone. These machines can not alter the form of an already developed & therefore solidified hailstone.

While there’s no scientific evidence to support the claimed effectiveness of these contraptions, the principle behind hail cannons predates scientific traction: traditionally, hail was warded off by ringing church bells.

On a different note, here’s another interesting quote from Tender Is The Night:

He went to the mail desk first—as the woman who served him pushed up with her bosom a piece of paper that had nearly escaped the desk, he thought how differently women use their bodies from men.